Rear Diffuser, Thoughts.?

[Silveredwings]

Oh. I thought a rear diffuser was when you ruffle the sheets after breaking wind. Nevermind.

[Bman83GL]

Silveredwings: You crack me up!

Formula2: As I was reading your post, I was going to direct you toward the Chapman "fan car," but I see that you have found this on your own. To JanGeo's point, a diffuser is only effective if you have a flat bottomed car. Even with a flat bottom, the surface friction is such that the air loses momentum as it heads from nose to tail. A gently expanding diffuser creates lower pressure at the rear, drawing the air that wants to slow under the car. While I imagine that most of this is for downforce, if you have efficient airflow under the car, it can reduce the amount of air going over the body. Depending on the Cd of the body and the smoothness of the underside, increased air under the car could actually reduce drag. If the velocity of this air stays high, it should not add ridiculous lift, either.
Some F1 teams have experimented with using the exhaust exits to stimulate the flow through the diffuser (per Simon McBeath). Due to the upredictable nature of the flow (goes away when you lift throttle) and the small positive effect, I think this was largely abandoned. I think that you'll find that you would need to spin the fans REALLY fast to have any beneficial effect.

All that said, I agree that for drag-reduction-only purposes, flat paneling the bottom will show greater effect than the diffuser. I think that adding the diffuser just makes the belly pan a bit more effective.

Thanks for posting the pic of the DBR9, by the way. Sorry for being lazy!
Bman

[omgwtfbyobbq]

Depending on how little pressure there is at the back of the car, directing air up into that area my reduce drag in the same way VGs supposedly reduce it. Which isn't to say that either works, but it's an interesting concept.

Diffused rear fascia
“We wanted to enhance the effect of the skin of the car being drawn down more tightly to the body and also reduce the visual weight of the rear end,” said Peters. “So we added a diffuser to the bottom of the rear fascia to enhance air flow and to add visual interest to the rear of 2005 Corvette.”

Four circular exhaust tips are integrated into the rear diffuser. The tips exit from the center of the diffuser and pick up the circular theme established by the four round taillamps. Framing the rear fascia with the black CHMSL, functional spoiler at the top and the black diffuser at the bottom produces a narrower cross section. In this way, the rear of 2005 Corvette is reduced both dimensionally and visually.

My guess is, the large the low pressure, the better a proper diffuser will work. Comparitively pointless for an Insight since it already terminates flow such that detachment is minimized, but probably decent for most hatches with flat backs/lots of area.

[omgwtfbyobbq]

Here's some more info, apparently Chrystler is using it on the Dodge Avenger concept. So it seems a properly designed diffuser has it's place on conventional high drag sedan designs, and probably any car made before 2000.

[omgwtfbyobbq]

If it works like VGs supposedly do, then alone I'm guessing it may reduce drag by ~5-20% depending on the older vehicle and mods that have already been done.

[mexellent]

Originally Posted by FormulaTwo

Im just wondering if the diffuser itself will reduce drag or increase downforce?

Undertrays rock anyway you cut it. Thats the beauty of it. Even if you were to put an aggressive undertray on your stock car, it would still help reduce drag of the car (while increasing downforce). Hell, just putting a flat panel underneath your car will still increase downforce and reduce drag at the same time(if angled correctly)!

We build cars with EXTREME aero packages. We know our shizzle.

[omgwtfbyobbq]

Oh sweetness! I was thinkin' about diffusers and was wonderin' if there could be a two stage kind of deal? Could we take something like this

with the air being directed upwards at a more extreme angle in order to fill in that huge low pressure area in the rear... combined with something like this

to smooth out the transition in the rear. For boxy cars, something that's kind of a two stage effect, with the ducting of the diffuser directing air to fill in that low pressure area while the fins/angle underneath it just direct the rest of the air so the flow seperates much farther downstream?

[omgwtfbyobbq]

As long as the area in front isn't increased, I don't think there'd be much to loose given how crappy most auto aero is. In fact, there's plenty of hot air already moving through the radiator. So heat would be added to it, as well as routing air flow that may normally be bouncing around the engine bay. Little fuzzy things can be kept out with mesh, which might hurt flow a bit, but would be better for all parties...

Thanks for the idea btw, keep 'em comin!

[thisisntjared]

Originally Posted by theclencher

Couple things about that diffuser: the air moving through the diffuser will have drag too unless enough heat energy is added to it. Also the height of the diffuser duct will displace the driver and other stuff upwards, thus increasing frontal area. Also rodents, raccoon, cats, chipmunks, and other various creatures might think that duct would be a great place to call home. So once again there ain't no free ride.

the ducts would not be enclosed.....

[omgwtfbyobbq]

That's the beauty of having a two layer system, I hope.
One takes the hot air that's flowed through the rad (or seperate areas) and dumps it out in the low pressure area behind the car, it's just there to even the pressure behind the car out. The other channeled section just directs the airflow such that it seperates as far back as possible, the advantage over just the channels being more air/higher pressure behind the car, so if done properly the redirected airflow from under the car seperates even farther back.